1. Field of the Invention
The present invention relates to a casting mold which is used to produce a cast product having a concave portion, e.g., a hole or the like, therein. It particularly relates to a construction of the casting mold.
2. Description of the Related Art
For instance, as set forth in Japanese Unexamined Utility Model Publication (KOKAI) No. 59-25,361, a core or a core pin has been used conventionally when producing a cast product which has a concave portion such as a hole or the like therein. The core or the core pin is subjected to a holding force which results from the shrinkage of a molten metal during solidification. Accordingly, it is difficult to remove the core or the core pin from the resulting cast product after cooling the cast product. Hence, in order to make the core or the core pin likely to be removed from the cast product, it is usually tapered gradually from wide to narrow in the direction toward its leading end.
When the cast products are produced by using the core or the core pin, they come to have a concave portion such as a hole or the like. However, the resulting concave portion is inevitably formed in a tapered hole whose inside diameter reduces from large to small in the direction toward its inner side. Thus, it is hard to make the resulting concave portion which has an identical inside diameter over its entire length. As a result, it is a routine practice to carry out machining on the inner periphery of the concave portion after the casting, thereby establishing an identical inside diameter over the entire length of the concave portion.
In particular, when die-casting aluminum or zinc, the molten aluminum or zinc is solidified rapidly at its surface where it is brought into contact with a mold. Consequently, at the surface where the molten aluminum or zinc is brought into contact with the mold, there is formed a healthy layer in which no bubbles are involved in a thickness of from about 0.7 to 1.0 mm. However, there exist blow holes in the deeper layer disposed under the healthy layer, because the molten aluminum or zinc is solidified at a slower rate in the deeper layer.
Thus, when the taper-holed concave portion formed by casting is machined, and especially when the concave portion has a long overall length, the machining allowance should be enlarged on the inner side of the concave portion so that it goes beyond the healthy layer. As a result, the blow holes come to be exposed to produce defects. For example, when a cast product is produced by using a core or a core pin having a draft angle of 1 degree and when the resulting concave portion has an overall length of 200 mm, the concave portion should be machined in excess of about 3.49 mm at its innermost portion. Accordingly, the concave portion is machined completely beyond the healthy layer. However, in view of the removability of the core or the core pin from the cast product, it is actually impossible to get rid of the draft taper, and accordingly it is inevitable to carry out the machining after the casting. Hence, there always exists the fear for machining the cast product beyond the healthy layer.
A casting process using a cast insert member has been known, in which casting is carried out after a cast insert member e.g., a liner or the like, formed independently is disposed in a cavity. In this process, there exists a fear for deforming a cast insert member, because a cylindrical liner, for instance, is deformed by the shrinkage force of a molten metal during solidification. Accordingly, casting is carried out after disposing a protective member in a cast insert member. If such is the case, there should be provided a clearance between the cast insert member and the protective member. Consequently, it is difficult to completely get rid of the deformation in the cast insert member. Further, in order to prevent the protective member from being stuck in the cast insert member due to the deformation in the cast insert member, the protective member should be formed in a configuration having a draft taper. Consequently, when the east insert member is deformed to conform to the configuration of the protective member, it is required to machine the inner periphery of the cast insert member after casting, and at the same time, there occur problems in that the machining has resulted in the partially fluctuating thickness in the cast insert member. Furthermore, there are produced defects which result from the molten metal invasion into the clearance between the cast insert member and the protective member.